Quantitative assessment of responses of the eyeball based on data from the Corvis tonometer
Introduction
The rapid advances in technology and, in particular, methods of image analysis and processing enable to perform a fully automatic measurement of intraocular pressure (although the measurement is not considered the gold standard). One such device is the Corvis tonometer [1], [2]. Apart from the measurement of intraocular pressure, it also allows for the measurement of other parameters of the eye [3]. The measurement method involves corneal flattening forced by an air puff—Fig. 1. Next, the corneal contour image analysis enables to measure the corneal thickness, deformation amplitude, applanation length, rate of corneal deflection and intraocular pressure [4], [5]. They are shown in Fig. 2. In addition, the information about applanation points, pachymetry, and others is provided [6], [7]. For example, determination of temporary states of corneal flattening during a convex/concave transition and vice versa enables to designate two moments in time [3], [5]. For these moments, the pressure value is read and then the mean value is calculated. This value after linear calibration is the IOP value. There are also other parameters of corneal deflection calculated in the Corvis tonometer (Fig. 2).
Modern methods of image analysis and processing allow for the measurement of a much larger number of parameters than those that are available in the Corvis tonometer and proprietary software. Their determination requires the use of the Roberts, Sobel and Prewitt edge detection methods [8]. Morphological operations such as opening and closing, also treated as conditional, are useful here. The created software is characterized by much greater functionality, full access to data and almost limitless possibility of further processing, in particular statistical processing. This is particularly important because in the current version of the Corvis software it is not possible to access data, e.g. the results of the designated corneal contour. Additionally, the contour is not designated in the full range of the cornea visible in the image.
Groups of studies related to the analysis of biomechanical parameters of the cornea for the ORA tonometer [9], [10] and the effect of patients’ age [11], [12], [13], [14], glaucoma [15] or wound healing [16] on the results obtained are known from the literature. Predictive numerical simulation of corneal biomechanical properties was considered in an equally interesting way [17]. The methods for measuring the ultrastructure of the corneal stroma are presented in paper [18]. There are also very interesting studies profiled to the analysis of the cornea in Brazilian patients [19] or children with the use of the ORA tonometer [20], [21]. There also exist publications which show the correlation between the hysteresis obtained using the tonometer and pachymetry [22]. By contrast, Congdon et al. [23] present hysteresis in correlation with glaucoma damage. The paper [24] deals with these issues (hysteresis) but in the Reichert ocular response analyser. Gatinel et al. [25] draw attention to the corneal hysteresis, resistance factor and topography. There is also a known group of studies related to the analysis of biomechanical parameters of the cornea in keratoconic eyes, [26], [27], or hypertension and glaucoma [28]. Also other examinations of the cornea are described which certainly contribute to the understanding of the biomechanical properties, for example, the corneal strip extensometry comparison presented in the paper [29]. In a review of the literature there are no studies related to the analysis of corneal images from the Corvis tonometer and, in particular, to the analysis of responses of the eyeball based on these images. Only in the paper [30] the authors show the division of the reaction to an air puff into the eyeball response and corneal response. The eyeball response itself is not analysed in the context of other biomechanical parameters, especially those available in the original software of the Corvis tonometer. The same applies to the paper [31] which shows the algorithm designed to acquire additional parameters from the Corvis tonometer, but without correlation analysis with the existing, well-known and aforementioned parameters. For this reason, this publication shows: (1) new biomechanical parameters of the eyeball response measured during corneal deformation; (2) correlation of the determined biomechanical parameters with the parameters measured by the original software of the Corvis tonometer; (3) analysis of results and evaluation of their practical usefulness.
Section snippets
Material
For the purposes of analysis, images with an M×N×I resolution of 200×576×140 pixel were acquired from the Corvis device in the source recording format *.cst. The patients ranged in age from 17 to 63 years. They were healthy (32 people including 16 women) or ill (16 people including 9 women). The ill ones were patients with AMD (age-related macular degeneration) or other diseases that cause abnormal pressure in the eye. For each patient, there were 140 images in a sequence, which for 96 eyes gave
Method
The proposed new method for the analysis of data from the Corvis tonometer was divided into two stages.
The first stage, namely image pre-processing, involved the analysis of data from the Corvis tonometer, which enabled the reconstruction of corneal shape changes and the separation of the eyeball response.
In the second stage, namely data processing, a detailed analysis of the response of the eyeball and its correlation with other parameters of the patient was performed.
Results
The results of a preliminary correlation between the features from w(1) to w(4) and from v(1) to v(14) obtained for 96 eyes are shown in the form of an image in Fig. 8. These results indicate that the highest correlation value (absolute values) is obtained for the correlation of v(1) with w(1), v(5) with v(4), v(11) with v(5), v(11) with v(14), v(11) with v(6) etc.—Table 2. The resulting correlation values will be subject to further description. The selection of only the highest correlation
Conclusions
The paper presents a fully automatic method for measuring responses of the eyeball calculated on the basis of data from the Corvis tonometer. In addition, a set of 4 different features, relevant from a practical point of view of the eyeball response assessment, was proposed. Moreover, the obtained results of responses of the eyeball were compared with the results of corneal reactions. The presented methodology of image acquisition and acquisition of individual parameters is not exhaustive of
Summary
The “air-puff” tonometers, include the Corvis, are a type of device for measuring intraocular pressure and biomechanics parameters. Additionally, one of the parameters obtainable from the Corvis tonometer using the software proposed by the authors is the response of the eyeball to an air puff. The paper attempts to analyse this response and its relationship with other parameters measured in the Corvis tonometer.
13,400 2D images were acquired from the Corvis device and analysed (32 healthy and
Abbreviations
ROI, region of interest; IOP, intraocular pressure.
Competing interests statement
The authors declare that they have no competing interests.
Authors’ contributions
RK suggested the algorithm for image analysis and processing, implemented it and analysed the images. EW, SW, AN, ST, AB, ZW performed the acquisition the images from Corvis and consulted the obtained results. All authors have read and approved the final manuscript.
Acknowledgements
No outside funding was received for this study.
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